Description of Related Art
One of the issues encountered when using an integrated heat spreader (IHS) is getting a balance between sufficient adhesion to the die, and a high enough heat flow to meet the cooling load of the die. To deal with this issue, different bonding materials have been tried with varying results. If the adhesion is insufficient, the IHS may spall off from the thermal interface material (TIM) and result in a yield issue or a field failure. Another issue encountered is achieving an acceptable IHS standoff from the die and the board to which the board is mounted. Because of various existing processes, a substrate-referenced process is used that may cause a significant variation in bond-line thickness (BLT) between the top of the die and the bonding surface of the IHS.
TIM BLT is maintained for mechanical reliability of the thermal interface during thermal cycling. Due to the difference in the coefficients of thermal expansion of the IHS and the die, there is a large amount of shear stress imposed on the TIM. Thicker bond lines assist the TIM to withstand these high stresses without failing.
TIM BLT is also an element in the thermal resistance of the thermal interface. A thinner TIM BLT can result in a lower thermal resistance. Due to these limits in TIM BLT, which can be required for acceptable package performance, TIM BLT must be tightly controlled.